1. Field of the Invention
The subject invention generally relates to a method of acquiring image data with a magnetic resonance imaging (MRI) system. More particularly, the invention relates to a method of acquiring image data using a sequence of radio frequency (RF) pulses for steady-state MRI.
2. Description of the Related Art
In magnetic resonance imaging (MRI), an MRI system includes a field magnet which is energized to produce a substantially homogeneous static magnetic (B0) field through a bore in which an object, typically a human body, is placed. The object includes a plurality of atomic nuclei, typically belonging to hydrogen atoms, within organic matter such as tissue, bones, etc. Each of the atomic nuclei exhibits an intrinsic angular momentum and spins about an axis. The magnetic moment of each of the atomic nuclei is known as a spin. The B0 field induces the spins to align according to a longitudinal axis defined by the B0 field. The MRI system includes an RF transmitter which temporarily applies an RF pulse to the object to rotate the spins away from the longitudinal axis. Thereafter, the spins precess (rotate) around the longitudinal axis with a frequency proportional to the local B0 field. Gradually, the spins realign to the longitudinal axis as a result of longitudinal relaxation (T1) and transverse relaxation (T2). In so doing, the spins release a detectable nuclear magnetic resonance (NMR) signal. The MRI system includes gradient coils which generate and temporarily apply magnetic field gradients to the object for determining the spatial location of the spins. The MRI system conventionally includes an RF receiver for receiving the NMR signal released by the spins. The MRI system processes the NMR signals to form part of an image corresponding to a scanned region of the object. The MRI system repeatedly applies RF pulses and magnetic field gradients along several slices of the scanned region to construct the entire image of the scanned region.
The sequence of RF pulses and magnetic field gradients may be repeated every “TR” milliseconds, where TR is an abbreviation for “sequence repetition time.” If TR is relatively shorter in duration than the transverse relaxation time T2, the spins will not have sufficient time to realign to the longitudinal axis prior to the application of each RF pulse. As a result, the spins generally settle into a dynamic steady-state, which is generally a function of tissue relaxation parameters (T1, T2) and of imaging sequence parameters, such as flip angle and sequence repetition time TR. Some of the advantages of steady-state MRI are that the image can be acquired rapidly, and that useful image contrast can be achieved with proper choice of RF and field gradient waveforms.
One common method of steady-state MRI imaging is balanced steady-state free precession (bSSFP) imaging. Balanced SSFP utilizes RF pulses combined with balanced magnetic field gradients, i.e., gradients which act on spins between consecutive RF pulses and preserve the phase of the spins existing before application of the gradient. Balanced SSFP can provide high signal-to-noise ratio (SNR) efficiency and useful T1-weighted and T2-weighted image contrast. Unfortunately, bSSFP imaging suffers from considerable signal loss as a consequence of local B0 inhomogeneity. Specifically, a significant loss of SNR and useful image contrast occurs in regions where the B0 frequency offset is an odd integer multiple of one-half of the inverse of the sequence repetition time, TR. Consequently, dark bands may be present in the image, which is undesirable and limits the application of bSSFP.
Accordingly, there remains an opportunity to provide an MRI imaging technique which mitigates the effects of local resonance frequency offset and eliminates signal variations.